The invention relates to the field of microphotonic light sources, and in particular to a planar multiwavelength optical power supply on a silicon platform.
Photonics technology revolutionized telecommunication systems in the later 1970s with the deployment of low-loss, single mode silica fibers and efficient, double heterostructure, single mode injection lasers. Long haul fiber optic systems were ultimately enabled with the development of erbium-doped fiber amplifiers (EDFA's), operating around the 1.55 μm low-loss fiber communication regime, dubbed the C-band. The EDFA's gain bandwidth is optimal within a 4000 GHz (30 nm) wide spectral window about 1.55 μm, thus permitting the eventual integration of wavelength division multiplexing (WDM) communications in the late 1990s.
Within the microelectronics industry, the continued drive of Moore's Law towards smaller circuit elements and denser chip architecture has now yielded to an intra-chip transmission limitation referred to as the interconnect bottleneck, an RC circuit delay occurring due to the smaller cross-section and closer spacing of conducting metal lines above the integrated circuit chip.
The recent growth of Metropolitan Area Networks (MAN), boosted by the rapid growth of the internet and a resulting demand for higher communication bandwidth, has given impetus to the development of micron-scale integrated photonic devices (microphotonics), creating a new generation range of faster, high yielding, higher functionality complex photonic devices. These micron-scale structures present a convergence of solutions for both the interconnect bottleneck problem, and the MAN requirement of low-cost high volume components.
Si-based microphotonics is a planar waveguide technology, combining all the necessary components for optical signal transmission and computing into a single optical chip, on a silicon wafer platform. These components include, lasers, switches, modulators, detectors, and channel add/drop filters. A significant reduction in cost and system size results from this dense planar integration, where all the optical components with different functionalities can be combined to yield an Integrated Optical Platform (IOP) or Optoelectronic Integrated Circuit (OEIC's) fully compatible with the widespread CMOS silicon microelectronics technology. This compatibility with CMOS technology leverages a powerful degree of planar processing experience in favor of designing high performance photonic structures. Silicon IOP would allow unprecedented information and computing capabilities appealing for a variety of technological markets.
The scientific and technological breakthrough of silicon microphotonics would consist of the demonstration of an efficient, CMOS compatible laser source for the silicon platform, enabling the integration of information processing silicon microelectronics with high bandwith transmitting silicon-based photonic structures. The invention proposed here provides a multi-frequency laser platform operating in the C-band telecommunication window with full CMOS compatibility.
Lasers and optical amplifiers are essential components of all optical circuits. Since light emission from silicon is an intrinsically inefficient process, current methods for realizing laser devices in integrated optical circuits involve expensive materials (mainly III-V semiconductor compounds) and deposition technologies (MBE, CVD) that cannot be easily integrated within CMOS silicon processing fabrication line.
However, passive silicon microphotonics, comprised of photonic structures performing light-guiding, routing and processing functions, has boomed during the last ten years. A near-complete operational set of photonics devices have been demonstrated: silicon based optical waveguides with extremely low losses and small curvature radii, tunable optical filters, fast switches (ns), fast optical modulators (GHz), fast photodetectors, integrated Ge photodetectors for 1.55 μm radiation. Micromechanical MEMS system and full band-gap photonic crystals have been demonstrated while switching systems are already commercial.
In the face of this passive component integration, the primary limitation to realizing a fully autonomous IOP is an efficient active Si-based device, such as a light emitting diode (LED) or a laser light source.
Silicon is an indirect band-gap material, resulting in light emission through a low probability phonon-mediated process (spontaneous recombination lifetimes in the ms range). In standard bulk silicon, competitive non-radiative recombination rates are much higher than this radiative rate, and the majority of excited excess electron-hole pairs recombine without photon emission, thus yielding a very low internal quantum efficiency (ηi≈10−6). In addition, fast non-radiative processes such as Auger or free carrier absorption severely prevent population inversion for silicon optical transitions at the high pumping rates needed to achieve optical amplification.
Despite of all these difficulties, during the 1990s several strategies have been investigated to cope with the intrinsically poor light emission yield of silicon.
Among the different approaches developed to overcome this material limitation, quantum confinement and rare earth doping of silicon have dominated the scientific efforts around active silicon microphotonics.
Due to the favorable modification of their optical properties, numerous silicon nanostructures, such as porous silicon, silicon nanocrystals embedded in an SiO2 matrix, and Si/SiO2 superlattices have been widely studied. The efficient, tunable and visible room temperature luminescence of all these structures has been ascribed to the recombination of quantum confined excitons which are self-trapped in a size dependent Si═O level at the interface between the silicon nanostructure and the SiO2 matrix.
Rare earth doping studies of Erbium (Er) doped crystalline silicon have demonstrated that Er can be excited in Si through electron-hole pair recombination or through impact excitation by high energy carriers, yielding Er-doped LEDs operating at room temperature. However, a very poor quantum efficiency and high intraband absorption rules out the possibility of realizing a laser or optical amplifier with such a photonic structure.
Er-doping of glass structures reveals an ideal approach since it produces an almost temperature independent emission line originating from an internal 4-f shell transition. Even though erbium doped SiO2 is used commercially to realize optical fiber amplifiers, the application of erbium-based structures in silicon microphotonics is limited so far by the small optical cross section of Er3+ transitions, and represents one of the major challenges of silicon-based microphotonics.
Recently, erbium doping of Si nanocrystals has been recognized as a hybrid method combining the promising features of both the described methods. Indeed, it has been demonstrated that Si nanocrystals in the presence of Er act as efficient sensitizers for the light emission of the rare earth. The effective excitation cross section of erbium ions in presence of Si nanocrystals is more than two orders of magnitudes higher with respect to the resonant absorption of a photon in a silica matrix while, in addition, non radiative de-excitation processes are strongly suppressed.
Recent observations of net optical gain at 1.54 μm with enhanced erbium emission cross section in Er-doped Si nanocluster sensitized waveguides, and the demonstration of efficient room temperature electro-luminescence from Er-silicon nanocrystal devices has opened the route towards the future fabrication of Si IOP devices based on Er amplification.
Room temperature continuous wavelength lasing action in the near infra-red and visible has been demonstrated by optical pumping of microdisk structures, comprised of III-V InP based materials. Enhanced photo-luminescence from Er-doped microdisks has been observed, establishing experimental groundwork for our realization of an Er-doped microdisk laser.
In addition, the fabrication of ultra-high Q toroid Er-doped microcavity on a silicon chip and the realization of planar Er-doped silica microdisk structures with extremely smooth edges is paving the way towards the realization of low noise, CMOS compatible erbium-based micro laser devices and optical amplifiers.